Many have questioned the facts recorded by several historians,
concerning the surprising effects of the burning mirrors of Archimedes,
by means of which the Roman galleys besieging Syracuse were consumed to
ashes. Descartes, in particular, discredited the story as fabulous; but
Kircher made many experiments with a view of testing its credibility. He
tried the effect of a number of plane mirrors; and, with five mirrors of
the same size, placed in a frame, he contrived to throw the rays
reflected from them to the same spot, at the distance of more than 100
feet; and by this means he produced such a degree of heat, as led him to
conclude that, by increasing their number, he could have set fire to
inflammable substances at a greater distance. He likewise made a voyage
to Syracuse, in company with his pupil Schottius, in order to examine
the place of the alleged transaction; and they were both of opinion,
that the galleys of Marcellus could not have been more than thirty paces
from Archimedes' mirrors.
M. Buffon also constructed a machine, consisting of a number of mirrors,
by which he seems to have revived the secret of Archimedes, and to have
vindicated the credit of history in this respect. His experiment was
first made with twenty-four mirrors, which readily set fire to
combustible matter composed of pitch and tow, and laid on a deal board
at the distance of seventy-two feet. He further pursued the attempt by
framing a kind of polyhedron, consisting of 168 pieces of plane
looking-glass, each six inches square; and by means of this machine,
some boards of beech-wood were set on fire at the distance of 150 feet,
and a silver plate was melted at the distance of 60 feet. This machine,
in the next stage of its improvement, contained 360 plane mirrors, each
eight inches long and six broad, mounted on a frame eight feet high and
seven broad. With twelve of these mirrors, light combustible matter was
kindled at the distance of twenty feet; with forty-five of them, at the
same distance, a large tin vessel was melted, and with 117, a thin piece
of silver. When the whole machine was employed, all the metals and
metallic minerals were melted at the distance of twenty-five and even of
forty feet. Wood was kindled in a clear sky at the distance of 210 feet.
M. Buffon afterwards constructed a machine which contained 400 mirrors,
each six inches square, with which he could melt lead and tin at the
distance of 140 feet.
But perhaps the most powerful burning mirror ever constructed, was that
of Mr. Parker, an eminent glass manufacturer of London; it was made in
the begining of this century by one Penn, an ingenious artisan of
Islington. He erected an outhouse at the bottom of his garden, for the
purpose of carrying on his operations, and at length succeeded in
producing, at a cost of £700, a burning lens of a diameter of three
feet, whose powers were astonishing. The most hard and solid substances
of the mineral world, such as platina, iron, steel, flint, &c., were
melted in a few seconds, on being exposed to its immense focus. A
diamond weighing ten grains, exposed to this lens for thirty minutes,
was reduced to six grains, during which operation it opened and foliated
like the leaves of a flower, and emitted whitish fumes; when closed
again, it bore a polish, and retained its form. Ten cut garnets, taken
from a bracelet, began to run into each other in a few seconds, and at
last formed one globular garnet. The clay used by Wedgewood to make his
pyrometric test ran in a few seconds into a white enamel; and several
specimens of lavas, and other volcanic productions, on being exposed to
the focus of the lens, yielded to its power.
A subscription was proposed in London to raise the sum of 700 guineas,
in order to indemnify the inventor for the expense he had incurred in
its construction, and retain it in England; but, through the failure of
the subscription, and other concurring circumstances, Mr. Parker was
induced to dispose of it to Captain Mackintosh, who accompanied Lord
Macartney in his celebrated embassy to China; and the mirror, much to
the loss and regret of European science, was left at Pekin.
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MAGNETIC CORRESPONDENCE IN THE
SEVENTEENTH CENTURY.
In one of Addison's contributions to the _Spectator_ (No. 241), we find
the following curious instance of what may almost be considered as the
foreshadowing of the electric telegraph. It is quoted from the writings
of Strada, the celebrated Roman Jesuit, who died in 1649. In his
"Prolusiones," a series of polished Latin essays upon rhetoric and
literature, he gives an account of a chimerical correspondence between
two friends, by the help of a certain loadstone, which had such virtue
in it, that if touched by two several needles, when one of the needles
so touched began to move, the other, though at ever so great a distance,
moved at the same time and in the same manner. He tells us that two
friends, being each of them possessed of these needles, made a kind of
dial-plate, inscribing it with twenty-four letters--in the same manner
as the hours of the day are marked upon the ordinary dial-plate. They
then fixed one of the needles on each of these plates, in such a manner
that it could move round without impediment so as to touch any of the
twenty-four letters. Upon their separating from one another into distant
countries, they agreed to withdraw themselves punctually into their
closets at a certain hour of the day, and to converse with one another
by means of this their invention. Accordingly, when they were some
hundred miles asunder, each of them shut himself up in his closet at the
time appointed, and immediately cast his eye upon his dial-plate. If he
had a mind to write anything to his friend, he directed his needle to
every letter that formed the words that he had occasion for--making a
little pause at the end of every word or sentence, to avoid confusion.
The friend, in the meanwhile, saw his own sympathetic needle moving of
itself to every letter which that of his correspondent pointed at. By
this means, they talked together across a whole continent, and conveyed
their thoughts to one another, in an instant, over cities or mountains,
seas or deserts.... In the meanwhile (adds the Essayist, playfully), if
ever this invention should be revived, or put in practice, I would
propose that upon the lovers' dial-plate there should be written, not
only the twenty-four letters, but several entire words which have always
a place in passionate epistles; as flames, darts, die, languish,
absence, Cupid, heart, eyes, hang, drown--and the like. This would very
much abridge the lover's pains in this way of writing a letter--as it
would enable him to express the most useful and significant words with a
single turn of the needle.
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